Bismuth–Carbon Anodes for Sodium‐Ion Batteries: Mechanistic Challenges and Future Design Directions

Sodium‐ion batteries (SIBs) are emerging as promising alternatives to lithium‐ion batteries for large‐scale energy storage owing to the abundance and low cost of sodium resources. Among candidate anode materials, bismuth (Bi) has attracted increasing interest due to its high theoretical capacity, moderate operating potential, and relatively high volumetric energy density. However, the practical implementation of Bi anodes is fundamentally limited by the intrinsic physicochemical characteristics of Na–Bi alloying. The reconstructive phase transformation during sodiation induces large volumetric changes, heterogeneous stress evolution, phase‐boundary‐controlled reaction kinetics, and unstable electrode–electrolyte interfaces, leading to structural degradation and performance decay during cycling. In this review, we critically examine the fundamental challenges governing Bi anodes and discuss how integrating Bi with carbon frameworks can regulate mechanical stability, electronic transport, and interfacial chemistry. Rather than providing a descriptive summary, this review establishes a mechanistic framework that links Na–Bi alloying chemistry to performance limitations across multiple length scales. Three tightly coupled governing processes, structural instability, phase‐boundary‐controlled kinetics, and dynamic interfacial evolution, are identified and analyzed in the context of Bi/C composite design. Emerging concepts, including electrolyte‐enabled self‐reformative architectures and electrode‐scale engineering, are further discussed to bridge the gap between laboratory studies and practical battery applications.